1. Field of the Invention
The field of the present invention relates generally to delivery vehicles used in a time release method for delivering an active ingredient and processes for producing such vehicles.
2. Description of the Prior Art
There are a wide variety of controlled release products presently being utilized in numerous applications, including pharmaceutical, agricultural and veterinary applications. Generally speaking, an active ingredient is released over time. The active ingredient may be contained in a variety of media, such as coated particles (e.g. sphere, aggregate, multiple coating or pill), solid solutions (e.g. beadlet, film, bandage or cube), compositions (e.g. sphere, tablet, pills or strip), containers (e.g. capsule, breakable ampule or capillary), and combinations (e.g. in a liquid, in a capsule or in a pill).
Microencapsulation is the most common process for preparing a time release delivery vehicle. Generally speaking, microencapsulation utilizes a coating to contain the active ingredient which is then released by rupture or dissolution of the coating. Alternatively, the coating or membrane may be semipermeable or porous to allow the active ingredient to diffuse out of the microencapsule.
U.S. Pat. No. 4,322,311 describes an encapsulation technique for producing semi-permeable or controlled porosity microcapsules. An active ingredient and a monomer in a first solution are emulsified in a hydrophobic solvent. A monomer complementary to the first monomer which is soluble in the hydrophobic solvent is added to the emulsion to initiate interfacial polymerization about the aqueous droplets. During the course of reaction, the affinity of the continuous phase for the first monomer is varied by adding a solvent to the continuous phase to vary its polarity. This promotes diffusion into the continuous phase resulting in a porous membrane. Since the patent describes the use of an amine monomer, some amine monomer may remain encapsulated within the microcapsule.
Another encapsulation technique is described in U.S. Pat. No. 4,444,699 wherein minute capsules are manufactured en masse. The process utilizes polycondensation of melamine with formaldehyde or in situ polymerization of methylol melamine or esterified methylol melamine, or a low molecular weight polymer thereof, in an aqueous vehicle and the reaction is conducted in the presence of polyelectrolyte material and certain salts. However, this process will typically leave some residue of formaldehyde which may pose a health problem.
Other examples of encapsulation techniques include, by way of example only, U.S. Pat. Nos. 4,324,683; 4,353,809; 4,353,888; 4,353,962; 4,391,909; 4,396,670; 4,407,957; 4,439,488; and 4,464,271. The microcapsules produced by these type of processes will typically possess limited mechanical strength and will release all of the active ingredients at once if the membrane is ruptured. The limited mechanical stability can create problems with incorporating the microcapsules into a medium and will also limit the shelf life of these delivery vehicles. In addition, the microcapsules will typically contain reactive groups which can create problems of chemical stability.
Another delivery vehicle for an active ingredient is described in U.S. Pat. No. 3,985,298 which utilizes a process to impregnate an active ingredient into and within a cellulosic polymer-liquid composite material as a part of or all of the liquid phase. The active ingredient is released from the gel matrix which shrinks or collapses as the active ingredient is removed. The gel structure is not mechanically strong and therefore it suffers some of the mechanical problems associated with microspheres.
Accordingly, there exists a need for an economical time-release delivery vehicle with high mechanical strength useful in a controlled-release application.